Heat cogeneration doesn't really improve efficiency of the methane generation process itself. Cogeneration refers to using the waste heat of a thermal plant to assist in some other facility. Using the waste heat of a gas plant to heat water in desalination is an example of cogeneration. So it does improve overall energy efficiency, but it assumes that there's a convenient source of heat next door. Wind and photovoltaics don't generate any significant amount of heat though, so there's no opportunity for cogeneration.